1. Field of the Invention
This invention relates to a lithium-containing complex metal oxide suited to a cathode electroactive material for lithium secondary cells, a preparation method thereof, a cathode electroactive material using the same, and a lithium secondary cell. More specifically, it relates to a lithium nickelate-based complex metal oxide which has excellent thermal stability and can be used as a cathode electroactive material in a lithium secondary cell (battery) wherein metal lithium or a lithium-carbon (lithium-graphite) intercalation compound is used as an anode electroactive material, thus making the cell has a high capacity and good cycle characteristics.
2. Related Background Art
The non-aqueous secondary cell disposed lithium or lithium compound as an anode has been expected to the cell having a high voltage and high energy density, and therefore, many studies have been proceeded. Widely known cathode electroactive materials for the secondary batteries with non-aqueous electrolyte solutions include the complex metal oxide comprising of lithium and other metal or metals such as lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide; metal oxides such as manganese dioxide, titanium disulfide, molybdenum disulfide, vanadium pentaoxide, and niobium pentaoxide; and chalcogens. These oxides and compounds have layer or tunnel crystal structures and are capable of repeating the reversible release and intercalation of lithium ions on the charge/discharge, respectively. Especially, active studies are being made on lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide for their use in the cathode electroactive material for lithium secondary cells with non-aqueous electrolyte solutions as four-volt (V) type cell. Lithium cobalt oxide, which is relatively easy to prepare, has already been put into practical use.
However, cobalt is a very expensive metal and also a strategic material, and the places of its origin are localized within certain regions; thus, there are problems such as the troubled supply due to changes in political situations or rising in the price. On the other hand, nickel and manganese are relatively inexpensive metals and their stable supply is possible. Lithium manganese oxide has a smaller capacity compared with lithium cobalt oxide and lithium nickel oxide, and has problems in cycle characteristics. Lithium nickel oxide has also some problems in its cycle characteristics. LiNiO.sub.2 results a change in its crystal structure from hexagonal to monoclinic, as it releases Li on the charge. This is believed to cause deterioration of the cycle characteristics. It has been found as a countermeasure to the deterioration that, if a part of the Ni in LiNiO.sub.2 is substituted with Co, there will be no change from hexagonal to monoclinic and thus the cycle characteristics will be improved. See, T. Ohzuka et al., J. Electrochem. Soc., 140, 1862 (1993) and S. Arai, S. Okada, H. Ohtsuka, and J. Yamamoto, Battery Technology (Denchi Gijyutsu), 7, 98 (1995).
When LiNiO.sub.2 is released Li through charging, NiO.sub.2 is formed. NiO.sub.2 is a very unstable compound, which generates heat while releasing oxygen. Accordingly, it is strongly desired that the thermal stability of LiNiO.sub.2 is improved.
It has been found that if a part of the Ni in LiNiO.sub.2 is substituted with Al, there will be a great improvement in its thermal stability. In this case, the charge capacity, however, lowers greatly. See, T. Ohzuka et al., J. Electrochem. Soc., 142, 4033 (1995).
Japanese Unexamined Patent Publication Sho 63-121,258 (1988) suggests a method to improve overpotential characteristics by substituting LiCoO.sub.2 with a variety of other metals. Also, in Japanese Unexamined Patent Publication Hei 5-242,891 (1993), it is suggested that if LiNi.sub.X Co.sub.Y O.sub.2 is further substituted with a variety of other metals, its discharge capacity will increase and that when Fe or Cu exists their thermal stability will be improved.
O. Zhong et al. have studied the synthesis and electrochemistry of LiAl.sub.Y Ni.sub.1-Y O.sub.2. See, O. Zhong and Ulchi von Sacken, J. Power Sources, 54, 221 (1995). Synthesis of LiAl.sub.Y Ni.sub.1-Y O.sub.2 was first attempted with a mixture of LiOH, NiO, and Al.sub.2 O.sub.3 (or Al (OH).sub.3), but the synthesis of a single-phase LiAl.sub.Y Ni.sub.1-Y O.sub.2 was unsuccessful, as the product was contaminated with Al.sub.2 O.sub.3 as an impurity. Thus, they changed the Al source to a metal Al powder (300 mesh) and, for the first time, succeeded in synthesizing the single-phase product. Nevertheless, its discharge capacity was as small as 104-148 mAh/g.